The invention relates to the field of transparent coatings for transparent objects such as eyeglass lenses, windows and the like, and refers particularly to a coating having an index of refraction essentially equal to that of the substrate that is coated.
Transparent plastic materials such as eyeglass lenses, television screen face plates and the protective coatings on photographic prints often are quite soft and are subject to becoming dull and hazy due to scratching and abrasion during use. Polycarbonate eyeglass lenses, for example, are strong and shatter resistant but also are relatively soft and susceptible to scratching. Television screen face plates similarly are made of flexible, shatter resistant plastic materials such as polycarbonate and poly (methylmethacrylate), and these also can be scratched or abraded.
Various coatings have been proposed for eyeglasses and other transparent plastic materials to reduce their propensity to become scratched and abraded. One such composition is shown in U.S. Pat. No. 4,378,250 (Treadway, et al.) granted Mar. 29, 1983. Other coatings are shown in U.S. Pat. No. 5,367.019 (Sawaragi), U.S. Pat. No. 4,855,180 (Kawamura), U.S. Pat. No. 4,895,767 (Mori et al.) and U.S. Pat. No. 4,719,146 (Hohage et al.)
In connection with lenses or other transparent substrates that bear protective coatings, if the refractive index of the coating varies even slightly from the refractive index of the substrate, visible and undesirable interference fringes will appear, particularly when the surface is viewed from an angle rather than p)erpendicular to the surface. Efforts have been made to produce coatings having high indices of refraction with the expectation that such coatings will avoid the interference fringe problem. Reference is made to Sawaragi et al., U.S. Pat. Nos. 5,367,019 and 5,314,947, referred to above. Such coatings have not been entirely satisfactory because they require thermal curing, which may lengthen production schedules and which also may not be appropriate for use with heat-sensitive substrates.
Thus, a coating is needed that is capable of producing a very hard and abrasion resistant surface upon curing, that has a refractive index that can be matched to the refractive index of the substrate to be coated, and that preferably can be cured quickly and easily.